Japanese Laid-open Patent Publication No. 183515/1989 discloses a highly water-absorptive fiber obtained by imparting an acryl fiber with high water absorption properties by the hydrolysis of the acryl fiber.
However, the defect is that when an acryl fiber is hydrolyzed, the fiber colors or that the percentage of physiological saline solution absoprtion decreases to about a half of the percentage of pure water absorption.
Japanese Laid-open Patent Publication No. 159405/1988 discloses a fiber formed of a copolymer which is produced from a vinyl monomer having a carboxylic acid group and a vinyl monomer having a hydroxyl group reactive with a carboxylic acid group and in which some of the carboxylic acid groups is converted to sodium salt. In this fiber, the percentage of physiological saline solution absorption does not decrease much as it is about 2/3 of the percentage of pure water absorption. However, the crosslinking reaction between the hydroxyl group and the carboxyl group proceeds slowly, and it is required to carry out the crosslinking reaction at a temparture of 180.degree. C. or higher, which increases the cost.